Simultaneous Production of Disk and Lobes: A Single-Wind MHD Model for the η Carinae Nebula

The luminous blue variable η Carinae is surrounded by a complex and highly structured nebula of ejected material. The best-studied and axisymmetric components of this outflow consist of bipolar lobes (the "homunculus") and an equatorial "skirt." Recent proper-motion measurements suggest that the skirt was ejected at the same time as the lobes, contrary to the assumptions of all current theoretical models for the formation of the nebula (which use the skirt to collimate stellar winds into lobes). We present a magnetohydrodynamic (MHD) stellar wind model that produces an outflowing disk and bipolar lobes in a single, steady state wind. The basic model consists of a wind from a rotating star with a rotation axis-aligned dipole magnetic field. The azimuthal component of the magnetic field, generated by stellar rotation, compresses the wind toward the equator and also toward the rotation axis, simultaneously producing an outflowing disk and jet. We use numerical MHD simulations to study the wind for various amounts of stellar rotation and to show a range of wind morphologies. In order to produce wide-angle lobes similar to the homunculus (which have roughly a 30° opening angle), a high-speed polar wind (with enhanced energy density) from the star is also required. In that case, the structure of the wind bears a remarkable resemblance to the skirt plus homunculus morphology of the η Car nebulae, and a significant fraction of the stellar angular momentum is carried away by the wind. Although the model assumes a steady state wind (rather than an eruption) and thermal wind driving (rather than radiation pressure), the structure of the wind is encouraging.